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server.go
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package zeroconf
import (
"fmt"
"log"
"math/rand"
"net"
"os"
"runtime"
"strings"
"sync"
"time"
"github.com/miekg/dns"
"golang.org/x/net/ipv4"
"golang.org/x/net/ipv6"
)
const (
// Number of Multicast responses sent for a query message (default: 1 < x < 9)
multicastRepetitions = 2
)
var defaultTTL uint32 = 3200
type serverOpts struct {
ttl uint32
}
func applyServerOpts(options ...ServerOption) serverOpts {
// Apply default configuration and load supplied options.
var conf = serverOpts{
ttl: defaultTTL,
}
for _, o := range options {
if o != nil {
o(&conf)
}
}
return conf
}
// ServerOption fills the option struct.
type ServerOption func(*serverOpts)
// TTL sets the TTL for DNS replies.
func TTL(ttl uint32) ServerOption {
return func(o *serverOpts) {
o.ttl = ttl
}
}
// Register a service by given arguments. This call will take the system's hostname
// and lookup IP by that hostname.
func Register(instance, service, domain string, port int, text []string, ifaces []net.Interface, opts ...ServerOption) (*Server, error) {
entry := newServiceEntry(instance, service, domain)
entry.Port = port
entry.Text = text
if entry.Instance == "" {
return nil, fmt.Errorf("missing service instance name")
}
if entry.Service == "" {
return nil, fmt.Errorf("missing service name")
}
if entry.Domain == "" {
entry.Domain = "local."
}
if entry.Port == 0 {
return nil, fmt.Errorf("missing port")
}
var err error
if entry.HostName == "" {
entry.HostName, err = os.Hostname()
if err != nil {
return nil, fmt.Errorf("could not determine host")
}
}
if !strings.HasSuffix(trimDot(entry.HostName), entry.Domain) {
entry.HostName = fmt.Sprintf("%s.%s.", trimDot(entry.HostName), trimDot(entry.Domain))
}
if len(ifaces) == 0 {
ifaces = listMulticastInterfaces()
}
for _, iface := range ifaces {
v4, v6 := addrsForInterface(&iface)
entry.AddrIPv4 = append(entry.AddrIPv4, v4...)
entry.AddrIPv6 = append(entry.AddrIPv6, v6...)
}
if entry.AddrIPv4 == nil && entry.AddrIPv6 == nil {
return nil, fmt.Errorf("could not determine host IP addresses")
}
s, err := newServer(ifaces, applyServerOpts(opts...))
if err != nil {
return nil, err
}
s.service = entry
s.start()
return s, nil
}
// RegisterProxy registers a service proxy. This call will skip the hostname/IP lookup and
// will use the provided values.
func RegisterProxy(instance, service, domain string, port int, host string, ips []string, text []string, ifaces []net.Interface, opts ...ServerOption) (*Server, error) {
entry := newServiceEntry(instance, service, domain)
entry.Port = port
entry.Text = text
entry.HostName = host
if entry.Instance == "" {
return nil, fmt.Errorf("missing service instance name")
}
if entry.Service == "" {
return nil, fmt.Errorf("missing service name")
}
if entry.HostName == "" {
return nil, fmt.Errorf("missing host name")
}
if entry.Domain == "" {
entry.Domain = "local"
}
if entry.Port == 0 {
return nil, fmt.Errorf("missing port")
}
if !strings.HasSuffix(trimDot(entry.HostName), entry.Domain) {
entry.HostName = fmt.Sprintf("%s.%s.", trimDot(entry.HostName), trimDot(entry.Domain))
}
for _, ip := range ips {
ipAddr := net.ParseIP(ip)
if ipAddr == nil {
return nil, fmt.Errorf("failed to parse given IP: %v", ip)
} else if ipv4 := ipAddr.To4(); ipv4 != nil {
entry.AddrIPv4 = append(entry.AddrIPv4, ipAddr)
} else if ipv6 := ipAddr.To16(); ipv6 != nil {
entry.AddrIPv6 = append(entry.AddrIPv6, ipAddr)
} else {
return nil, fmt.Errorf("the IP is neither IPv4 nor IPv6: %#v", ipAddr)
}
}
if len(ifaces) == 0 {
ifaces = listMulticastInterfaces()
}
s, err := newServer(ifaces, applyServerOpts(opts...))
if err != nil {
return nil, err
}
s.service = entry
s.start()
return s, nil
}
const (
qClassCacheFlush uint16 = 1 << 15
)
// Server structure encapsulates both IPv4/IPv6 UDP connections
type Server struct {
service *ServiceEntry
ipv4conn *ipv4.PacketConn
ipv6conn *ipv6.PacketConn
ifaces []net.Interface
shouldShutdown chan struct{}
shutdownLock sync.Mutex
refCount sync.WaitGroup
isShutdown bool
ttl uint32
}
// Constructs server structure
func newServer(ifaces []net.Interface, opts serverOpts) (*Server, error) {
ipv4conn, err4 := joinUdp4Multicast(ifaces)
if err4 != nil {
log.Printf("[zeroconf] no suitable IPv4 interface: %s", err4.Error())
}
ipv6conn, err6 := joinUdp6Multicast(ifaces)
if err6 != nil {
log.Printf("[zeroconf] no suitable IPv6 interface: %s", err6.Error())
}
if err4 != nil && err6 != nil {
// No supported interface left.
return nil, fmt.Errorf("no supported interface")
}
s := &Server{
ipv4conn: ipv4conn,
ipv6conn: ipv6conn,
ifaces: ifaces,
ttl: opts.ttl,
shouldShutdown: make(chan struct{}),
}
return s, nil
}
func (s *Server) start() {
if s.ipv4conn != nil {
s.refCount.Add(1)
go s.recv4(s.ipv4conn)
}
if s.ipv6conn != nil {
s.refCount.Add(1)
go s.recv6(s.ipv6conn)
}
s.refCount.Add(1)
go s.probe()
}
// SetText updates and announces the TXT records
func (s *Server) SetText(text []string) {
s.service.Text = text
s.announceText()
}
// TTL sets the TTL for DNS replies
//
// Deprecated: This method is racy. Use the TTL server option instead.
func (s *Server) TTL(ttl uint32) {
s.ttl = ttl
}
// Shutdown closes all udp connections and unregisters the service
func (s *Server) Shutdown() {
s.shutdownLock.Lock()
defer s.shutdownLock.Unlock()
if s.isShutdown {
return
}
if err := s.unregister(); err != nil {
log.Printf("failed to unregister: %s", err)
}
close(s.shouldShutdown)
if s.ipv4conn != nil {
s.ipv4conn.Close()
}
if s.ipv6conn != nil {
s.ipv6conn.Close()
}
// Wait for connection and routines to be closed
s.refCount.Wait()
s.isShutdown = true
}
// recv4 is a long running routine to receive packets from an interface
func (s *Server) recv4(c *ipv4.PacketConn) {
defer s.refCount.Done()
if c == nil {
return
}
buf := make([]byte, 65536)
for {
select {
case <-s.shouldShutdown:
return
default:
var ifIndex int
n, cm, from, err := c.ReadFrom(buf)
if err != nil {
continue
}
if cm != nil {
ifIndex = cm.IfIndex
}
_ = s.parsePacket(buf[:n], ifIndex, from)
}
}
}
// recv6 is a long running routine to receive packets from an interface
func (s *Server) recv6(c *ipv6.PacketConn) {
defer s.refCount.Done()
if c == nil {
return
}
buf := make([]byte, 65536)
for {
select {
case <-s.shouldShutdown:
return
default:
var ifIndex int
n, cm, from, err := c.ReadFrom(buf)
if err != nil {
continue
}
if cm != nil {
ifIndex = cm.IfIndex
}
_ = s.parsePacket(buf[:n], ifIndex, from)
}
}
}
// parsePacket is used to parse an incoming packet
func (s *Server) parsePacket(packet []byte, ifIndex int, from net.Addr) error {
var msg dns.Msg
if err := msg.Unpack(packet); err != nil {
// log.Printf("[ERR] zeroconf: Failed to unpack packet: %v", err)
return err
}
return s.handleQuery(&msg, ifIndex, from)
}
// handleQuery is used to handle an incoming query
func (s *Server) handleQuery(query *dns.Msg, ifIndex int, from net.Addr) error {
// Ignore questions with authoritative section for now
if len(query.Ns) > 0 {
return nil
}
// Handle each question
var err error
for _, q := range query.Question {
resp := dns.Msg{}
resp.SetReply(query)
resp.Compress = true
resp.RecursionDesired = false
resp.Authoritative = true
resp.Question = nil // RFC6762 section 6 "responses MUST NOT contain any questions"
resp.Answer = []dns.RR{}
resp.Extra = []dns.RR{}
if err = s.handleQuestion(q, &resp, query, ifIndex); err != nil {
// log.Printf("[ERR] zeroconf: failed to handle question %v: %v", q, err)
continue
}
// Check if there is an answer
if len(resp.Answer) == 0 {
continue
}
if isUnicastQuestion(q) {
// Send unicast
if e := s.unicastResponse(&resp, ifIndex, from); e != nil {
err = e
}
} else {
// Send mulicast
if e := s.multicastResponse(&resp, ifIndex); e != nil {
err = e
}
}
}
return err
}
// RFC6762 7.1. Known-Answer Suppression
func isKnownAnswer(resp *dns.Msg, query *dns.Msg) bool {
if len(resp.Answer) == 0 || len(query.Answer) == 0 {
return false
}
if resp.Answer[0].Header().Rrtype != dns.TypePTR {
return false
}
answer := resp.Answer[0].(*dns.PTR)
for _, known := range query.Answer {
hdr := known.Header()
if hdr.Rrtype != answer.Hdr.Rrtype {
continue
}
ptr := known.(*dns.PTR)
if ptr.Ptr == answer.Ptr && hdr.Ttl >= answer.Hdr.Ttl/2 {
// log.Printf("skipping known answer: %v", ptr)
return true
}
}
return false
}
// handleQuestion is used to handle an incoming question
func (s *Server) handleQuestion(q dns.Question, resp *dns.Msg, query *dns.Msg, ifIndex int) error {
if s.service == nil {
return nil
}
switch q.Name {
case s.service.ServiceTypeName():
s.serviceTypeName(resp, s.ttl)
if isKnownAnswer(resp, query) {
resp.Answer = nil
}
case s.service.ServiceName():
s.composeBrowsingAnswers(resp, ifIndex)
if isKnownAnswer(resp, query) {
resp.Answer = nil
}
case s.service.ServiceInstanceName():
s.composeLookupAnswers(resp, s.ttl, ifIndex, false)
default:
// handle matching subtype query
for _, subtype := range s.service.Subtypes {
subtype = fmt.Sprintf("%s._sub.%s", subtype, s.service.ServiceName())
if q.Name == subtype {
s.composeBrowsingAnswers(resp, ifIndex)
if isKnownAnswer(resp, query) {
resp.Answer = nil
}
break
}
}
}
return nil
}
func (s *Server) composeBrowsingAnswers(resp *dns.Msg, ifIndex int) {
ptr := &dns.PTR{
Hdr: dns.RR_Header{
Name: s.service.ServiceName(),
Rrtype: dns.TypePTR,
Class: dns.ClassINET,
Ttl: s.ttl,
},
Ptr: s.service.ServiceInstanceName(),
}
resp.Answer = append(resp.Answer, ptr)
txt := &dns.TXT{
Hdr: dns.RR_Header{
Name: s.service.ServiceInstanceName(),
Rrtype: dns.TypeTXT,
Class: dns.ClassINET,
Ttl: s.ttl,
},
Txt: s.service.Text,
}
srv := &dns.SRV{
Hdr: dns.RR_Header{
Name: s.service.ServiceInstanceName(),
Rrtype: dns.TypeSRV,
Class: dns.ClassINET,
Ttl: s.ttl,
},
Priority: 0,
Weight: 0,
Port: uint16(s.service.Port),
Target: s.service.HostName,
}
resp.Extra = append(resp.Extra, srv, txt)
resp.Extra = s.appendAddrs(resp.Extra, s.ttl, ifIndex, false)
}
func (s *Server) composeLookupAnswers(resp *dns.Msg, ttl uint32, ifIndex int, flushCache bool) {
// From RFC6762
// The most significant bit of the rrclass for a record in the Answer
// Section of a response message is the Multicast DNS cache-flush bit
// and is discussed in more detail below in Section 10.2, "Announcements
// to Flush Outdated Cache Entries".
ptr := &dns.PTR{
Hdr: dns.RR_Header{
Name: s.service.ServiceName(),
Rrtype: dns.TypePTR,
Class: dns.ClassINET,
Ttl: ttl,
},
Ptr: s.service.ServiceInstanceName(),
}
srv := &dns.SRV{
Hdr: dns.RR_Header{
Name: s.service.ServiceInstanceName(),
Rrtype: dns.TypeSRV,
Class: dns.ClassINET | qClassCacheFlush,
Ttl: ttl,
},
Priority: 0,
Weight: 0,
Port: uint16(s.service.Port),
Target: s.service.HostName,
}
txt := &dns.TXT{
Hdr: dns.RR_Header{
Name: s.service.ServiceInstanceName(),
Rrtype: dns.TypeTXT,
Class: dns.ClassINET | qClassCacheFlush,
Ttl: ttl,
},
Txt: s.service.Text,
}
dnssd := &dns.PTR{
Hdr: dns.RR_Header{
Name: s.service.ServiceTypeName(),
Rrtype: dns.TypePTR,
Class: dns.ClassINET,
Ttl: ttl,
},
Ptr: s.service.ServiceName(),
}
resp.Answer = append(resp.Answer, srv, txt, ptr, dnssd)
for _, subtype := range s.service.Subtypes {
resp.Answer = append(resp.Answer,
&dns.PTR{
Hdr: dns.RR_Header{
Name: subtype,
Rrtype: dns.TypePTR,
Class: dns.ClassINET,
Ttl: ttl,
},
Ptr: s.service.ServiceInstanceName(),
})
}
resp.Answer = s.appendAddrs(resp.Answer, ttl, ifIndex, flushCache)
}
func (s *Server) serviceTypeName(resp *dns.Msg, ttl uint32) {
// From RFC6762
// 9. Service Type Enumeration
//
// For this purpose, a special meta-query is defined. A DNS query for
// PTR records with the name "_services._dns-sd._udp.<Domain>" yields a
// set of PTR records, where the rdata of each PTR record is the two-
// label <Service> name, plus the same domain, e.g.,
// "_http._tcp.<Domain>".
dnssd := &dns.PTR{
Hdr: dns.RR_Header{
Name: s.service.ServiceTypeName(),
Rrtype: dns.TypePTR,
Class: dns.ClassINET,
Ttl: ttl,
},
Ptr: s.service.ServiceName(),
}
resp.Answer = append(resp.Answer, dnssd)
}
// Perform probing & announcement
//TODO: implement a proper probing & conflict resolution
func (s *Server) probe() {
defer s.refCount.Done()
q := new(dns.Msg)
q.SetQuestion(s.service.ServiceInstanceName(), dns.TypePTR)
q.RecursionDesired = false
srv := &dns.SRV{
Hdr: dns.RR_Header{
Name: s.service.ServiceInstanceName(),
Rrtype: dns.TypeSRV,
Class: dns.ClassINET,
Ttl: s.ttl,
},
Priority: 0,
Weight: 0,
Port: uint16(s.service.Port),
Target: s.service.HostName,
}
txt := &dns.TXT{
Hdr: dns.RR_Header{
Name: s.service.ServiceInstanceName(),
Rrtype: dns.TypeTXT,
Class: dns.ClassINET,
Ttl: s.ttl,
},
Txt: s.service.Text,
}
q.Ns = []dns.RR{srv, txt}
// Wait for a random duration uniformly distributed between 0 and 250 ms
// before sending the first probe packet.
timer := time.NewTimer(time.Duration(rand.Intn(250)) * time.Millisecond)
defer timer.Stop()
select {
case <-timer.C:
case <-s.shouldShutdown:
return
}
for i := 0; i < 3; i++ {
if err := s.multicastResponse(q, 0); err != nil {
log.Println("[ERR] zeroconf: failed to send probe:", err.Error())
}
timer.Reset(250 * time.Millisecond)
select {
case <-timer.C:
case <-s.shouldShutdown:
return
}
}
// From RFC6762
// The Multicast DNS responder MUST send at least two unsolicited
// responses, one second apart. To provide increased robustness against
// packet loss, a responder MAY send up to eight unsolicited responses,
// provided that the interval between unsolicited responses increases by
// at least a factor of two with every response sent.
timeout := time.Second
for i := 0; i < multicastRepetitions; i++ {
for _, intf := range s.ifaces {
resp := new(dns.Msg)
resp.MsgHdr.Response = true
// TODO: make response authoritative if we are the publisher
resp.Compress = true
resp.Answer = []dns.RR{}
resp.Extra = []dns.RR{}
s.composeLookupAnswers(resp, s.ttl, intf.Index, true)
if err := s.multicastResponse(resp, intf.Index); err != nil {
log.Println("[ERR] zeroconf: failed to send announcement:", err.Error())
}
}
timer.Reset(timeout)
select {
case <-timer.C:
case <-s.shouldShutdown:
return
}
timeout *= 2
}
}
// announceText sends a Text announcement with cache flush enabled
func (s *Server) announceText() {
resp := new(dns.Msg)
resp.MsgHdr.Response = true
txt := &dns.TXT{
Hdr: dns.RR_Header{
Name: s.service.ServiceInstanceName(),
Rrtype: dns.TypeTXT,
Class: dns.ClassINET | qClassCacheFlush,
Ttl: s.ttl,
},
Txt: s.service.Text,
}
resp.Answer = []dns.RR{txt}
s.multicastResponse(resp, 0)
}
func (s *Server) unregister() error {
resp := new(dns.Msg)
resp.MsgHdr.Response = true
resp.Answer = []dns.RR{}
resp.Extra = []dns.RR{}
s.composeLookupAnswers(resp, 0, 0, true)
return s.multicastResponse(resp, 0)
}
func (s *Server) appendAddrs(list []dns.RR, ttl uint32, ifIndex int, flushCache bool) []dns.RR {
v4 := s.service.AddrIPv4
v6 := s.service.AddrIPv6
if len(v4) == 0 && len(v6) == 0 {
iface, _ := net.InterfaceByIndex(ifIndex)
if iface != nil {
a4, a6 := addrsForInterface(iface)
v4 = append(v4, a4...)
v6 = append(v6, a6...)
}
}
if ttl > 0 {
// RFC6762 Section 10 says A/AAAA records SHOULD
// use TTL of 120s, to account for network interface
// and IP address changes.
ttl = 120
}
var cacheFlushBit uint16
if flushCache {
cacheFlushBit = qClassCacheFlush
}
for _, ipv4 := range v4 {
a := &dns.A{
Hdr: dns.RR_Header{
Name: s.service.HostName,
Rrtype: dns.TypeA,
Class: dns.ClassINET | cacheFlushBit,
Ttl: ttl,
},
A: ipv4,
}
list = append(list, a)
}
for _, ipv6 := range v6 {
aaaa := &dns.AAAA{
Hdr: dns.RR_Header{
Name: s.service.HostName,
Rrtype: dns.TypeAAAA,
Class: dns.ClassINET | cacheFlushBit,
Ttl: ttl,
},
AAAA: ipv6,
}
list = append(list, aaaa)
}
return list
}
func addrsForInterface(iface *net.Interface) ([]net.IP, []net.IP) {
var v4, v6, v6local []net.IP
addrs, _ := iface.Addrs()
for _, address := range addrs {
if ipnet, ok := address.(*net.IPNet); ok && !ipnet.IP.IsLoopback() {
if ipnet.IP.To4() != nil {
v4 = append(v4, ipnet.IP)
} else {
switch ip := ipnet.IP.To16(); ip != nil {
case ip.IsGlobalUnicast():
v6 = append(v6, ipnet.IP)
case ip.IsLinkLocalUnicast():
v6local = append(v6local, ipnet.IP)
}
}
}
}
if len(v6) == 0 {
v6 = v6local
}
return v4, v6
}
// unicastResponse is used to send a unicast response packet
func (s *Server) unicastResponse(resp *dns.Msg, ifIndex int, from net.Addr) error {
buf, err := resp.Pack()
if err != nil {
return err
}
addr := from.(*net.UDPAddr)
if addr.IP.To4() != nil {
if ifIndex != 0 {
var wcm ipv4.ControlMessage
wcm.IfIndex = ifIndex
_, err = s.ipv4conn.WriteTo(buf, &wcm, addr)
} else {
_, err = s.ipv4conn.WriteTo(buf, nil, addr)
}
return err
} else {
if ifIndex != 0 {
var wcm ipv6.ControlMessage
wcm.IfIndex = ifIndex
_, err = s.ipv6conn.WriteTo(buf, &wcm, addr)
} else {
_, err = s.ipv6conn.WriteTo(buf, nil, addr)
}
return err
}
}
// multicastResponse is used to send a multicast response packet
func (s *Server) multicastResponse(msg *dns.Msg, ifIndex int) error {
buf, err := msg.Pack()
if err != nil {
return fmt.Errorf("failed to pack msg %v: %w", msg, err)
}
if s.ipv4conn != nil {
// See https://pkg.go.dev/golang.org/x/net/ipv4#pkg-note-BUG
// As of Golang 1.18.4
// On Windows, the ControlMessage for ReadFrom and WriteTo methods of PacketConn is not implemented.
var wcm ipv4.ControlMessage
if ifIndex != 0 {
switch runtime.GOOS {
case "darwin", "ios", "linux":
wcm.IfIndex = ifIndex
default:
iface, _ := net.InterfaceByIndex(ifIndex)
if err := s.ipv4conn.SetMulticastInterface(iface); err != nil {
log.Printf("[WARN] mdns: Failed to set multicast interface: %v", err)
}
}
s.ipv4conn.WriteTo(buf, &wcm, ipv4Addr)
} else {
for _, intf := range s.ifaces {
switch runtime.GOOS {
case "darwin", "ios", "linux":
wcm.IfIndex = intf.Index
default:
if err := s.ipv4conn.SetMulticastInterface(&intf); err != nil {
log.Printf("[WARN] mdns: Failed to set multicast interface: %v", err)
}
}
s.ipv4conn.WriteTo(buf, &wcm, ipv4Addr)
}
}
}
if s.ipv6conn != nil {
// See https://pkg.go.dev/golang.org/x/net/ipv6#pkg-note-BUG
// As of Golang 1.18.4
// On Windows, the ControlMessage for ReadFrom and WriteTo methods of PacketConn is not implemented.
var wcm ipv6.ControlMessage
if ifIndex != 0 {
switch runtime.GOOS {
case "darwin", "ios", "linux":
wcm.IfIndex = ifIndex
default:
iface, _ := net.InterfaceByIndex(ifIndex)
if err := s.ipv6conn.SetMulticastInterface(iface); err != nil {
log.Printf("[WARN] mdns: Failed to set multicast interface: %v", err)
}
}
s.ipv6conn.WriteTo(buf, &wcm, ipv6Addr)
} else {
for _, intf := range s.ifaces {
switch runtime.GOOS {
case "darwin", "ios", "linux":
wcm.IfIndex = intf.Index
default:
if err := s.ipv6conn.SetMulticastInterface(&intf); err != nil {
log.Printf("[WARN] mdns: Failed to set multicast interface: %v", err)
}
}
s.ipv6conn.WriteTo(buf, &wcm, ipv6Addr)
}
}
}
return nil
}
func isUnicastQuestion(q dns.Question) bool {
// From RFC6762
// 18.12. Repurposing of Top Bit of qclass in Question Section
//
// In the Question Section of a Multicast DNS query, the top bit of the
// qclass field is used to indicate that unicast responses are preferred
// for this particular question. (See Section 5.4.)
return q.Qclass&qClassCacheFlush != 0
}